I want to be able to update a value in a table to the existing value plus
or minus a value set in a variable without having to read in the current
value first. Any help would be appreciated.

The desired action could be described like the following query:

UPDATE tablename SET columnname='currentvalue_in_column +-
value_in_variable' WHERE key='key_variable';



Thank You,
Christopher A. Goodfellow
Director of Information Technology
Tealuxe, Inc.
Phone: 508 520 7887 ex:22
Fax: 508 528 8999
www.tealuxe.com
tea for all



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Christopher A. Goodfellow wrote:
I would use SELECT FOR UPDATE, perform the calculation on
the client side, then UPDATE the column, and finally COMMIT
the transaction. See the description of the "FOR UPDATE"
clause of the SELECT command (in PostgreSQL's online docs),
which ensures that the updated row is locked, so there is
no danger using SELECT + UPDATE.

HTH.

Best regards
Oliver

--
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.

"anyone new to programming should be kept as far from C++ as
possible; actually showing the stuff should be considered a
criminal offence" -- Jacek Generowicz

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TIP 9: the planner will ignore your desire to choose an index scan if your
joining column's datatypes do not match

Christopher A. Goodfellow wrote:
Ah, sorry, I misunderstood you. I thought you were
concerned about the possibility of concurrent updates,
i.e. the necessity of locking. That's why my brain
immediately went into the SELECT FOR UPDATE direction.
:-)

Well, yes, of course you can update a column based on
the old value of that column. The UPDATE command
accepts arbitrary expressions for the new value which
may reference the old one. There's even an example
which does exactly that (on the PostgreSQL's doc page
on the UPDATE command):

http://www.postgresql.org/docs/7.4/s...ql-update.html

Best regards
Oliver

--
Oliver Fromme, secnetix GmbH & Co KG, Oettingenstr. 2, 80538 München
Any opinions expressed in this message may be personal to the author
and may not necessarily reflect the opinions of secnetix in any way.

"The ITU has offered the IETF formal alignment with its
corresponding technology, Penguins, but that won't fly."
-- RFC 2549

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